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154 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 which is less risky than extrapolating empirical data. The second type of value stems from the uncertain timetable for dealing with global warming, which is quite aggressive in some parts of the world and creates a need for detailed, credible peer-reviewed results of carefully designed exper- iments. In both cases the world auto industry—OEM’s and component suppliers—have decided to pool resources and share the cost of producing such information. The results of the European RACE consortium’s efforts were produced by member companies and shared internally, and later placed in the public domain [132]. To enhance acceptance, a broader- based cooperative research program is now being under- taken by SAE and conducted at the University of Illinois. That 18-month effort involves a rigorous experimental evaluation of several competing systems, with extensive instrumentation under identical operating conditions [141]. The systems are: baseline production R-134a; advanced R-134a; transcritical CO2; and propane (R-290) with a secondary loop containing a non-flammable coolant. Laboratory experiments—carefully designed and docu- mented—are one important step along the path to commercializing any new technology. Designing and retooling to accommodate the high pressures in CO2 refrigeration systems, and dealing with safety concerns are generic issues to be faced by any companies involved in a transition to CO2 systems. The auto industry is positioned to benefit from existence of captive vehicle fleets, which can provide test-beds for confirming the conclusions drawn from laboratory experiments. On the other hand the industry is beset with a variety of logistical concerns related to the worldwide service infrastructure: the need for technicians to handle not only the new technology but also its overlap with conventional systems. Maintenance problems are more prominent in vehicles than in stationary applications because of the need for more extensive spatial, mechanical and electrical integration of the air-conditioning unit with other subsystems (severe packaging constraints; compressor mechanical drive; and the need for reheating dehumidified air for defogging). Moreover, the auto industry is accustomed to producing cars capable of operating in almost any climate. Therefore, investigations of advanced cooling applications are being accompanied by experiments in heating mode, because many of today’s efficient cars reject too little waste heat to the engine coolant to keep cars comfortable in severe winter conditions. 8.2. Automotive heating Modern cars with efficient fuel-injection engines often have insufficient waste heat for heating of the passenger compartment in the winter season. The long heating-up period and slow defroster action is unacceptable both in terms of safety and comfort. Supplementary heating is therefore necessary, and one attractive solution may be to operate the air conditioning system as a heat pump. CO2 systems have special benefits in heat pump mode, since high capacity and COP can be achieved also at low ambient temperature and with high air supply temperature to the passenger compartment. The first results of CO2 heat pump experiments were obtained by running an auto air-conditioning prototype system in reverse [142 – 144]. Although the cross-counter- flow interior heat exchangers were far from ideal, the data shown in Fig. 46 show the essential features of an automotive heat pump: capacity is highest at startup when it is needed most; the capacity is at least three times higher than what could be obtained from an electric resistance or friction heater due to the high heat pumping efficiency; and capacity and efficiency (heating performance factor, HPF) decline slowly due to reduced volumetric and isentropic compressor efficiencies at higher temperature lift, as the car warms up and heat becomes available from the engine coolant. These initial results have proven quite valuable in guiding the design and development of improved components for next-generation systems. Next-generation prototypes will need to address such operational issues as defogging. One reason heat pumps are not currently employed in automobiles is that R-134a has disadvantages as a heat pump fluid (e.g. large compressor displacement; air in-leakage in compressor shaft seal at subatmospheric suction pressures). Regardless of the fluid, however, air- to-air heat pumps present substantial technological chal- lenges that have not yet been addressed: the outdoor heat exchanger will accumulate frost, and perhaps ice as water is splashed on it from the road. Very little is known about frosting and condensate drainage from ultra-compact microchannel heat exchangers, and what is known suggests that the difficulties may be substantial [143–146]. Residen- tial heat pumps defrost by switching into air conditioning Fig. 46. Automotive heat pump performances at different indoor and outdoor conditions.PDF Image | CO2 Vapor Compression Systems
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